Of course this is not in Gurbani because it happened in 1699. Actually, IMHO, this does not matter. What we understand from the formation of the Khalsa is the same eaither way. It is the act of giving your head, without any doubt or reservation, with complete surrender to Guru Ji. That is all we need to know: the state of consciousness of the Panj Piyare, which, as Khalsa, is ours as well.